1. Field of Invention
The present invention relates to liquid crystal display (LCD) structures and methods of forming them. More particularly, the present invention relates to LCD structures of integrating a black matrix and a color filter into a substrate having a control circuit thereon and methods of forming the same.
2. Description of Related Art
Liquid crystal display (LCD) has many advantages over other conventional types of displays including high picture quality, small volume occupation, lightweight, low voltage driven and low power consumption. Hence, LCD is widely used in small portable televisions, mobile telephones, video recording units, notebook computers, desktop monitors, projector televisions and so on. LCD gradually replaces conventional cathode ray tube (CRT) as a mainstream display unit.
The main part of LCD is liquid crystal (LC) unit composed of two parallel transparent substrates and LC sealed therein. The mainstream of the LCD is thin film transistor (TFT) LCD. The fabrication processes of a TFT-LCD can be divided into four parts: TFT array process, color filter (CF) process, LC cell assembly process, liquid crystal module (LCM) process.
The TFT array process is used to fabricate a TFT substrate. Each TFT respectively aligns with one pixel electrode. The CF process is used to fabricate color filter substrate. A color filter layer composed of different color filter sheet is on the color filter substrate, and a black matrix layer surrounds each color filter sheet.
The LC cell assembly process is used to parallel assembles TFT substrate and CF substrate, and bead spacers spread between them to maintain a fixed distance, i.e. cell gap, between TFT substrate and CF substrate. LC is injected into the cell gap then the injection opening is sealed. Basically, each pixel electrode is respectively corresponding to one color filter sheet, and the black matrix layer covers on TFTs and metal lines that connect different TFT.
The LCM process is used to attach a polarizer to a panel, and electrically connect driver IC and panel circuit. Then a reflector and a back-light are assembled on the panel. After a burn-in step, the LCM process is finished.
Generally speaking, the direction of liquid crystal molecule axes, which are controlled by TFT, determines whether each pixel is pervious to light or not. The color of each pixel is determined by the color of color filter sheet. For example, when light passes through a red color filter sheet, a red spot is shown on the panel. Mixing red, green and blue colors can show full-color images.
Since the molecular axes of liquid crystal molecules, which are between pixel electrodes and color filter sheets, have to be controlled precisely, the color filter and the TFT substrate must be aligned precisely. The tolerable error of alignment is only within several micron meters.
The heat coefficients of color filter substrate and TFT substrate are different, hence the precise alignment of pixel electrodes and color filter sheets is hard to achieve. Decreasing the product yield and increasing the production cost are caused by this problem. With growing size of substrates, the problem is getting serious. Therefore, light leakage and coin mura on panel is produced. But for increasing the size of black matrix to increase the alignment precision, the color contrast and brightness of LCD is decreased.
Another problem is the random distribution of bead spacers. The bead spacers may block some of the pixel area. Hence the aperture ratio of LCD is decreased. For smaller pixel area or for high-resolution panel, the problem is more serious.